Ultrafiltration fractions of the aqueous extracts of lymphoid tissue (30,000-50,000 daltons) have been shown to contain a totally cell-specific and non-cytotoxic inhibitory activity toward the transformation of mouse and human lymphocytes in vitro. This inhibitory activity is both thermolabile and destroyed by either trypsin or chymotrypsin incubation; ribonuclease (trypsin-free) and deoxyribonuclease have no effect upon this inhibitory activity. Injection of this inhibitory fraction from spleen or thymus will prolong by 250% to 300% the graft survival of skin allografts from C3H mice into C57 mice. Presumably further treatment with this lymphocyte inhibitor would further prolong the survival of these grafts in vivo. This material will also inhibit the release of MIF and skin reactive factor (lymphokines) from immunologically stimulated lymphocytes in vitro. This immunosuppressant activity of the ultrafiltration fractions from aqueous extracts of lymphoid tissue is believed to be due to a specific and endogenous negative feedback inhibitor of mitosis, or 'chalone.' We have developed techniques for the preparation of sufficiently large amounts of this ultrafiltered fraction and its subsequent purification via Concanavalin A-agarose column chromatography after ultrafiltration to be able to study the in vivo immunosuppressive properties of various doses of this material against both heart and skin transplants. This technology, which has been developed in conjunction with Millipore Corporation, will permit the essentially pilot plant production quantities of these lymphocyte chalone concentrates to be prepared. Experiments are currently underway to explore the lymphopenia produced in vivo by the administration of large quantities of spleen- derived lymphocyte chalone concentrate. The correlation of this lymphopenia with the immunosuppression of graft rejection is currently under study.